Transcriptomic Analyses Reveal Leaf Colour Changes and L-theanine Accumulation in Variegated Tea

Abstract

Background Camellia sinensis ‘Yanlinghuayecha’ (YHC) is a variegated mutant developed recently in China. To dissect the physiological and molecular mechanisms of leaf variegation, we compared the leaf pigmentation, cellular ultrastructure, amino acid content, and transcriptome between the albino (A), mosaic (M), and green (G) sectors.Results The contents of photosynthetic pigments were significantly lower in sector A and higher in sector G than in sector M. Chloroplasts with well-organized thylakoids were found only in the mesophyll cells of the G sector but not in those of the A sector. The A sector had a significantly higher content of total and free amino acids. In particular, the levels of theanine, glutamate, and alanine in the A sector were higher than those in the G sector. Transcriptomics analysis showed that a total of 44,908 unique transcripts were identified. Comparing the differentially expressed genes (DEGs) in the three sectors, we conducted an in-depth study on chloroplast biogenesis, chlorophyll biosynthesis, and theanine synthesis pathways. The expression of CsPPOX in “porphyrin and chlorophyll metabolism” was significantly downregulated in the A sector. CsLHCB6 in “Photosynthesis - antenna proteins” and CsSCY1 in “Protein processing in endoplasmic reticulum”, both of which were associated with chloroplast biogenesis, were significantly downregulated in the A sector. The expression of CsTS1 was notably upregulated in the A sector.Conclusion Taken together, variegation alters the gene activities involved in chloroplast biogenesis, and our results suggest that leaf colour change in the A sector incorporates three aspects compared with that in the G sector: (1) Decreased CsPPOX expression slows the rate of chlorophyll synthesis, resulting in a decrease in chlorophyll content; (2) downregulated expression of CsLHCB6 and CsSCY1 inhibits chloroplast biogenesis, decreasing thylakoid morphogenesis and grana stacking; and (3) the metabolic flow of glutamate changes, possibly from chlorophyll biosynthesis to theanine biosynthesis. The accumulation of precursor synthetic substances and the high expression of CsTS1 generates a high theanine content. These analyses provide valuable insights into variegation in tea plants with regard to leaf colour change and L-theanine accumulation.